The world remains dependent on petroleum-derived feedstreams as the source of energy, especially for use as transportation fuels. These feedstreams are produced from crude oils using various refinery processes in complex commercial petroleum refineries. While many useful products are produced in these refineries, undesirable side streams, such as acid gases (CO2 and H2S), are also produced. An increase of the concentration of CO2, a greenhouse gas, in the atmosphere due to carbon emissions is expected to occur unless energy systems incorporate carbon emission reduction technology. CO2 capture and sequestration, along with reduced carbon content of fuels and improved efficiency of energy production and use, are considered to be viable ways to stabilize and ultimately reduce the concentration of greenhouse gases.
CO2 capture in industrial processes is typically done by aqueous amine scrubbing, which generally involves contacting the CO2-containing gas stream with an aqueous solution of one or more simple amines such as diethanolamine, monoethanolamine, and the like. The amines chemically react with CO2 in a reversible manner to form one or more of carbamate, ammonium bicarbonate, and ammonium carbonate species. Another acid gas scrubbing technology is the so-called “Rectisol Wash” process that uses an organic solvent, typically methanol, at subzero temperatures. SELEXOL™ is another conventional acid gas removal process wherein a proprietary solvent is used into which acid gases are dissolved from a hydrocarbon stream. Another commercial process for removing acid gases from hydrocarbon streams is a process marketed under the tradename PURISOL™ which uses N-methyl-2-pyrrolidone (NMP) as a solvent for absorbing acid gases.
Another gas scrubbing technology in commercial use is the Flexsorb™ process. This process removes acid gases from natural or flue gas streams using aqueous solutions of sterically hindered simple amines in a temperature swing process operating at temperatures of about 35 to about 85° C., or higher. The steric hindrance of the amine is used to suppress CO2 uptake in order to best favor selective H2S removal. Since most process gas streams enter the scrubbing stage at elevated temperatures, much energy is consumed in heat transfer to cool the feed gas to the CO2/H2S absorption temperature. A process that could operate in a non-aqueous-phase environment, and at higher temperatures, would be highly desirable, as would a process that is more selective for CO2 as opposed to H2S.
Polymeric amines (polyamines) and amine-bearing polymers are also used as CO2 sorbents. Polymeric amines have much larger molecular weights than simple small-molecule amines and are therefore less volatile. Smaller sorbent loss is achieved through vaporization over the lifetime of the process for CO2 sorption processes performed in the liquid phase using temperature swing. Polymeric amines may also be used in neat form as bulk sorbents.
Polyethylenimine, also known as poly(aziridine), is a polyamine of interest as a CO2 sorbent. Poly(aziridine) is the polymeric form of aziridine (ethylenimine), a three membered ring cyclic amine, and is typically hyperbranched in microstructure. Branchy poly(aziridine)s are amorphous, viscous liquids at room temperature. Poly(aziridine) provides a number of advantages as a sorbent polymer, particularly a very high density of amine sites per weight (one amine site for every two carbon atoms). This high density of potentially reactive amine sites gives poly(aziridine) a very high potential CO2 uptake on a weight basis and renders it a potentially very efficient sorbent.
CO2 capture has also been performed using solid-phase sorbents in which simple amines are supported on the surface of, and/or within the pores of, silica and zeolite materials which may already function as active physisorbents for CO2. These solid sorbents are advantageous over liquid-phase sorbents in that the CO2-containing gas stream can be contacted directly with the sorbent without need of a liquid medium in a variety of configurations such as particles in fixed beds, fibers, etc. Both small molecule and polymeric amines, including poly(aziridine), have been used in supported form to absorb CO2 from moist gases at moderate temperatures (≦120° C.). Polymeric amines which are physically impregnated into, or chemically attached to a support, offer a greater number of amine binding sites than impregnated small-molecule amines due to the multifunctional (multi-amine) nature of the polymer species. Pre-made poly(aziridine)s have been impregnated and/or covalently attached to supports such as mesoporous silicas (MCM-41, SBA-15), other polymers, and carbon nanotubes and used for reversible CO2 capture. Liquid poly(aziridine)s on poly(methyl methacrylate) substrates are utilized by NASA for CO2 removal on the Space Shuttle. As compared to CO2 absorption by neat poly(aziridine)s, synergistic effects between zeolitic supports and the poly(aziridine) can raise the absorption capacities of the polymer by a factor of about 2 to 3.
Poly(aziridine) can be introduced to an inorganic support in multiple fashions. Pre-polymerized poly(aziridine) may be physically impregnated onto the support by mixing in solution followed by evaporation of the solvent. This technique is non-selective in siting the polymer molecules on both the surface of the support and within its pores. Large, high-molecular-weight polymers may be too large to enter the pores, or may block entrance to them, giving non-optimal surface area of the chemical sorbent or destroying the synergistic effects provided by physisorption of the CO2 within the pores of the mesoporous silica. In situ polymerization of aziridine in the presence of the inorganic support provides a method for efficiently introducing polymer molecules into pores, for example into those of mesoporous silicas, as described in Rosenholm, J. et al. Chem. Mater. 2007, 19, 5023; Chem. Commun. 2006, 3909. The chemisorptions of CO2 using high surface area SBA-15 silica supports, such as MCM-41 mesoporous silica, that contain in-situ grown poly(aziridine)s are described in United States Patent Application Publication US2007/0149398A1, which is incorporated herein by reference. The poly(aziridine)s are prepared from aziridine monomer within the pores of the support utilizing the pendant hydroxyl, carboxylic acid, or other similar groups of the silica as initiating moieties for polymerization in the presence of a catalytic amount of a Brønsted acid such as acetic acid. In these materials, the loading (amount) of the branchy poly(aziridine)s is greater than in materials prepared by simple impregnation of pre-grown poly(aziridine)s, and higher CO2 sorption capacities are achieved. These materials also offer good long-term stability as a result of the covalent attachment of the poly(aziridine) to the support.
While there exists several commercial processes for the sequestration of CO2 from various hydrocarbon streams, there is still a need in the art for improvement, particularly with respect to costs, effectiveness, reversibility and toxicity. In particular, what is needed is a method to manipulate the properties of polyamine sorbents to provide improved safety, versatility, and CO2 sorption capacity. For nanotube-supported poly(aziridine)s, CO2 absorption efficiencies of only about ≦15% are known, suggesting that there is much room for manipulation of the polymer microstructure to improve CO2 uptake. The accessibility of amine sites in supported poly(aziridine)s for CO2 sorption has in some instances also been shown to be less than for monomeric amines. Furthermore, poly(aziridine) is prepared from a very volatile monomer, aziridine, having a high toxicity.